...grinding back and forth taking, losing and retaking planets?
Its rare in my DA metaverse games to be in the planet taking/retaking situtation. Usually the AI have colonized the region with less than maximum initial population - therefore their nearby developed planets are also low on population; their shortage of troops will hinder their retake invasions.
Only side effect is after gaining all the colonies you run into a defecit which lasts about a couple months.
Demolish all the research buildings and factories which are not on bonus tiles if the defecit will be more than a couple of months. I do this when a race has surrendered to me and I am low on BCs.
Amazingly they accepted and i broke their space in two. I don't know if it was a glitch or a bug or just pure luck...
Quicker than building influence starbases.
You might have caught them at a bad time (sudden military weakness / too busy elsewhere / lost election / loss of resource starbases / and that sort of thing). Plenty of stats to sift through - I think you would find a non-glitch reason somewhere.